This invention relates to complex oxide catalysts and production process of acrylic acid. More particularly, the invention relates to complex oxide catalysts suitable for use in producing acrylic acid from acrolein by vapor-phase catalytic oxidation reaction, and to a producing process of acrylic acid from acrolein using said catalysts.
A large number of improved catalysts for preparing acrylic acid through vapor phase catalytic oxidation reaction of acrolein have been proposed. For example, Japanese Patent Publication No. 12129/69 described a catalyst formed of molybdenum, vanadium and tungsten; Publication No. 11371/74, that formed of molybdenum, vanadium, copper, tungsten and chromium; Publication No. 25914/75, that formed of molybdenum and vanadium; and Laid-open (Kokai) Patent Application, Kokai No. 85091/77, that formed of molybdenum, vanadium, copper and at least one element of antimony and germanium.
However, these conventional catalysts are not fully satisfactory for industrial working, because of such defects that the yield of the object product, i.e., acrylic acid, is insufficient and deterioration rate in activity is high, leading to short catalyst life. Therefore, development of catalysts which excel in stability and enable acrylic acid production at high yield over prolonged periods has been in demand.
The applicant has disclosed the catalysts containing molybdenum, vanadium and alkaline earth metals in Laid-open (Kokai) Patent Application, Kokai No. 117419/74, which catalysts, however, are still open to improvements in respect of acrylic acid yield and catalyst life.
Accordingly, one of the objects of the present invention is to provide complex oxide catalysts, in particular, those which are suitable for producing acrylic acid through vapor phase catalytic oxidation of acrolein.
Another object of the present invention is to provide a process for preparing acrylic acid at high yield over prolonged periods, by oxidizing acrolein in the presence of catalyst at vapor phase with molecular oxygen or a molecular oxygen-containing gas.
We have discovered that the desired catalysts excelling in activity, selectivity and also catalyst life and which exhibit stable performance over prolonged periods can be obtained in the occasion of preparing a complex oxide catalysts expressed by the following general formula (1):
MoaVbWcCudAeBfCgDhEiOxxe2x80x83xe2x80x83(1) 
(in which the components and their ratios are as later identified),
when a compound containing both components B and C is used as at least a part of supply sources of components B and C; and that the use of this catalyst enables production of acrylic acid from acrolein at high yield over prolonged periods. Based on these discoveries the present invention is completed.
Namely, the present invention relates to a complex oxide catalyst which is expressed by the following general formula (1):
MoaVbWcCudAeBfCgDhEiOxxe2x80x83xe2x80x83(1) 
(in which Mo is molybdenum; V is vanadium, W is tungsten, Cu is copper, A is at least an element selected from antimony, niobium and tin; B is at least an element selected from alkaline earth metals; C is at least an element selected from silicon, aluminum, titanium and zirconium; D is at least an element selected from phosphorus, tellurium, cerium, lead, arsenic and zinc; E is at least an element selected from Group IA and Group IIIb elements of the periodic table, boron, iron, bismuth, cobalt, nickel and manganese; and O is oxygen; a, b, c, d, e, f, g, h, i and x denote the atomic ratios of Mo, V, W, Cu, A, B, C, D, E and O, respectively; and where a=12, 2xe2x89xa6bxe2x89xa615, 0xe2x89xa6cxe2x89xa610, 0 less than dxe2x89xa66 (preferably 0.05xe2x89xa6dxe2x89xa66), 0xe2x89xa6exe2x89xa66, 0 less than fxe2x89xa610 (preferably 0.01xe2x89xa6fxe2x89xa610), 0 less than gxe2x89xa610 (preferably 0.01xe2x89xa6gxe2x89xa610), 0xe2x89xa6hxe2x89xa65, 0xe2x89xa6ixe2x89xa65, and x is a numerical value determined by the extents of oxidation of the other elements)
which is characterized in that a compound containing both of the components B and C is used as at least a part of the supply sources of components B and C at the time of the catalyst preparation.
Those complex oxide catalysts which are represented by the general formula (1) are per se known as disclosed in said Kokai No. 117,419/74. In the complex oxide catalysts of the invention, preferably antimony and tin are used as the component A; magnesium, calcium, strontium and barium, as the component B; silicon and aluminum, as the component C; phosphorus, tellurium and zinc; as the component D; sodium, potassium, iron, cobalt, nickel and boron, as the component E; respectively.
The characteristic feature of the invention lies in the use of a compound containing both components B and C (which is hereafter referred to as a B/C components-containing compound) as at least a part of supply sources (starting compounds) of components B and C in the occasion of preparing the complex oxide catalysts of the present invention. The reason why the complex oxide catalysts of excellent performance are obtained through such a practice is not yet clear. At the present time we presume that whereby improved stability of the component B contributes to the better performance, while the scope of this invention should never be restricted by this presumption.
The volume ratio of a B/C components-containing compound in the supply sources of components B and C (i.e., total volume of the starting material of component B and that of component C) is 0.5/1 to 1/1, preferably 0.8/1 to 1/1, in terms of the atomic ratio. In particular, it is preferred to supply the total amount of the component B in the catalyst from a B/C components-containing compound.
As the supply sources for Mo, V, W, Cu, components A, D and E; any compounds which contain the named individual elements and which produce the corresponding oxides upon calcination can be used.
As B/C components-containing compounds, any of marketed compounds (preferably oxides) which contain both components B and C can be used as they are. Examples of such marketed oxides include barium aluminate (2BaO.Al2O3.5H2O), magnesium silicate (Mg2Si3O8.5H2O), calcium silicate (CaSiO3), barium titanate (BaTiO3), strontium titanate (SrTiO3), calcium titanate (CaTiO3), calcium zirconate (CaZrO3) and the like. Other than these commercial products, oxides containing both components B and C can be prepared through, for example, the following procedures: {circle around (1)} dissolve or disperse a component Bxe2x80x94containing compound and a component C-containing compound in water, dewater and give such treatments like drying, and thereafter calcine at prescribed temperatures, preferably at 500-2000xc2x0 C.; {circle around (2)} thoroughly mix a component Bxe2x80x94containing oxide with a component Cxe2x80x94containing oxide and calcine the mixture at prescribed temperatures, preferably at 500-2000xc2x0 C.; {circle around (3)} calcine a B/C componentsxe2x80x94containing compound at prescribed temperatures, preferably at 500-2000xc2x0 C.
Said B/C componentsxe2x80x94containing compounds are preferably used in pulverized state to an average particle diameter of not greater than 200 xcexcm, more advantageously not greater than 100 xcexcm, and most advantageously, not greater than 50 xcexcm.
Where either of the components B and C comprises more than one element, it is sufficient for the B/C componentsxe2x80x94containing compound to contain at least one of the elements as the component B or C. The B/C componentsxe2x80x94containing compound may also contain a component other than the components B and C, e.g., component E. Obviously, such a compound is useful also as a supply source of the component E.
The complex oxide catalysts of the invention can be prepared by the methods generally practiced for preparing this kind of complex oxide catalysts, except that a B/C componentsxe2x80x94containing compound is used as at least a part of the supply sources of the components B and C.
Shape of the complex oxide catalysts of the invention is not critical. They may be molded into any optional forms such as ring, sphere, column, tablet and the like, with an average diameter of 1-15 mm, preferably 3-10 mm. In preparing the catalysts, those well known additives having the effect of improving the strength and attrition resistance of catalysts, such as inorganic fibers, e.g., glass fiber or various whiskers may be added. Also for controlling the catalyst properties with good reproducibility, additives generally known as powder binder such as ammonium nitrate, cellulose, starch, polyvinyl alcohol, stearic acid and the like may be used.
While the complex oxide catalysts of the invention are each useful by itself (as molded catalyst), they are preferably used in the form supported on inert carriers such as alumina, silica-alumina, silicon carbide, silicon nitride, titanium dioxide, aluminium sponge and the like (as supported catalyst). In the latter case, suitable supported ratio (%) of the complex oxide expressed by the general formula (1) ([(weight of the complex oxide)/(weight of the inert carrier+weight of the complex oxide)]xc3x97100) is 10-70%, preferably 15-50%.
Production of acrylic acid from acrolein according to the present invention can be performed by any of known methods, except that one of the so far described complex oxide catalysts should be used as the catalyst. The apparatus and operating conditions in carrying out the production are not critical. That is, as the reactor, an ordinary fixed bed reactor, fluidable bed reactor or moving bed reactor can be used, and the reaction can be carried out under the conditions conventionally employed for production of acrylic acid from acrolein through vapor phase catalytic oxidation reaction. For example, a gaseous mixture of 1-15 volume % of acrolein, 0.5-25 volume % of oxygen, 1-30 volume % of steam and 20-80 volume % of an inert gas like nitrogen, is contacted with a complex oxide catalyst of the invention at temperatures ranging from 200 to 400xc2x0 C., under a pressure of 0.1-1 MPa and at a space velocity of 300-5,000 hxe2x88x921 (STP) to produce acrylic acid.
Besides such gaseous mixtures of acrolein, oxygen and inert gas, acrolein-containing gaseous mixtures which are obtained through direct oxidation of propylene may also be used as the starting gas, if necessary after adding air or oxygen and steam. Presence of such side products as acrylic acid, acetic acid, carbon oxide and propane or unreacted propylene in the acrolein-containing gaseous mixtures obtained upon direct oxidation of propylene is in no way detrimental to the catalysts used in the process of the invention.
According to the invention, high-activity and high-performance catalysts are obtainable with good reproducibility. Moreover, because the complex oxide catalysts of the invention maintain the high activity levels over prolonged periods, acrylic acid can be stably produced at high yields over prolonged periods according to the process of the invention.